std: Migrate to the new libc

* Delete `sys::unix::{c, sync}` as these are now all folded into libc itself
* Update all references to use `libc` as a result.
* Update all references to the new flat namespace.
* Moves all windows bindings into sys::c

src/librustc_llvm/lib.rs

@@ -16,6 +16,8 @@
 #![allow(dead_code)]
 #![allow(trivial_casts)]
 
+#![cfg_attr(stage0, allow(improper_ctypes))]
+
 #![crate_name = "rustc_llvm"]
 #![unstable(feature = "rustc_private", issue = "27812")]
 #![staged_api]